1. Field of the Invention
The present invention relates to a carbon-based negative electrode material which can be used with an electrolyte containing propylene carbonate as a main ingredient and a secondary battery which contains such an electrolyte.
2. Description of the Related Art
For a negative electrode of a secondary battery, a carbon-based material such as graphite is used. Graphite has a layer structure where layers of six-membered rings each with an inter-atomic distance of about 0.14 nm are stacked at intervals of about 0.33 nm. A secondary battery is charged and discharged utilizing occlusion of lithium ions from a positive electrode into a gap between the layers.
In the case of using graphite for a negative electrode, the use of an electrolyte containing propylene carbonate (PC) as a main ingredient, which is a non-aqueous electrolyte, causes decomposition of the electrolyte and promotes separation of graphite, so that lithium ions cannot be inserted and extracted. For this reason, an electrolyte containing ethylene carbonate (EC) as a main ingredient has been used. However, EC has a low permittivity and becomes solid at room temperature. Further, when lithium salt is dissolved in EC, the viscosity increases and the ionic conductivity decreases. That is why it is difficult to use EC alone as a solvent of an electrolyte.
Thus, a technique which allows the use of an electrolyte containing PC as a main ingredient even in the case of using graphite for a negative electrode has been studied. Patent Document 1 discloses that when a coating layer containing any of an ionic conductive polymer, a water-soluble polymer, and an alkali metal compound is provided over a surface of a carbon negative electrode, propylene carbonate can be used for an electrolyte. Example 1 in Patent Document 1 discloses that a negative electrode coating including natural graphite was applied to Cu foil to form a carbon negative electrode and then a coating layer was applied to a surface of the carbon negative electrode and dried.
In Patent Document 2, description is given of, for example, insufficient control over the reactivity of a surface of a carbon negative electrode to propylene carbonate, referring to Patent Document 1 as a reference, and a technique is disclosed in which a spherical material obtained by attaching a polymer material to a spherical graphite particle as a negative electrode active material is used as a negative electrode material in combination with an electrolyte containing carbonate having an unsaturated bond and propylene carbonate.